Phase behavior and chemical composition of borehole fluids are used to help estimate the viability of some hydrocarbon reservoirs. For example, the concentration of gaseous components such as carbon dioxide, hydrogen sulfide and methane in borehole fluids are indicators of the economic viability of a hydrocarbon reservoir. The concentrations of various different gasses may be of interest for different reasons. For example, CO2 corrosion and H2S stress cracking are leading causes of mechanical failure of production equipment. CH4 is of interest as an indicator of the calorific value of a gas well. It is therefore desireable to be able to perform fluid analysis quickly, accurately, reliably, and at low cost.
A variety of techniques and equipment are available for performing fluid analysis in a laboratory. However, retreiving samples for laboratory analysis are time consuming. Further, some characteristics of borehole fluids change when brought to the surface due to the difference in environmental conditions between a borehole and the surface and other factors. For example, because hydrogen sulfide gas readily forms non-volatile and insoluble metal sulfides by reaction with many metals and metal oxides, analysis of a fluid sample retrieved with a metallic container can result in an inaccurate estimate of sulfide content. This presents a technological problem because known fluid analysis techniques that can be used at the surface are impractical in the borehole environment due to size limitations, extreme temperature, extreme pressure, presence of water, and other factors. Another technological problem is isolation of gases, and particular species of gas, from the borehole fluid.
The technological problems associated with detection of gas in fluids have been studied in this and other fields of research. For example, US20040045350A1, US20030206026A1, US20020121370A1, GB2415047A, GB2363809A, GB2359631A, US6995360B2, US6939717B2, W02005066618A1, W02005017514A1, W02005121779A1, US20050269499A1, and US20030134426A1 describe an electrochemical method for H2S detection using membrane separation. US20040045350A1, GB2415047A, and GB2371621A describe detecting gas compounds by combining infrared spectrophotometry and a membrane separation process. US20060008913 A1 describes the use of a perfluoro-based polymer for oil-water separation in microfluidic system.